Royalex: Royalex is made out of layered ABS plastic (bowling balls are made from ABS plastic) with a 1/2" foam core. Some canoes are made with as many as fourteen layers, while others only have a few. The outer-hull should have a vinyl coating to protect the ABS, which is very sensitive to the suns UV rays. Knowing this you need to ask how many layers of ABS are in Royalex, as not all canoes in this category are created equal.
Royalite, which is a sub-set of Royalex is probably the material of choice today for a middle of the line canoe. Lightweight, extremely durable, and extremely slippery, it is an ideal material for whitewater running. Because it has a foam core, it is natural buoyant, and because it is multiple layers of material it is very rigid yet has a strong memory. If you plan to wrap a canoe around the rocks, this should be your material of choice.
Royalex and Royalite is not the perfect material, however. Because it requires a vinyl skin, dragging it across branches and rocks will cause a lot of abrasion on the hull. Royalex and Royalite does not do well to long term sun exposure and will require indoor storage. The cells in the foam core contract and expand, and temperature extremes in storage (above or below "normal" North American temperature extremes) can stress the material. Royalex and Royalite is more difficult to repair if the hull is breeched then polyethylene, but damage to the hull of that extent is less likely.
Royalex is best the best all around material whether your plans are flat water, to extreme whitewater conditions. Royalite in particular is very lightweight, and can be half the weight of an equal sized polyethylene canoe making it ideal for portage. If you plan to use a canoe with only limited frequency and want to store it outside, you might be better off considering aluminum or an alloy canoe.
Kevlar: Kevlar was made famous by it's application in bullet proof vests. Extremely light weight and extremely durable, Kevlar is an excellent choice if you plan to operate in more extreme conditions. It is even lighter than Royalite, but it can be very expensive. Some of the best deals on Kevlar canoes can be found in Canada, where the US exchange rate plays into the cost of manufacturing.
Kevlar is a weaved material, similar to a cloth fabric, and appears honey-gold in it's raw form. This material weave is soaked in resin, shaped and cured to create the canoe hull. Kevlar frizzes if it gets damaged so the hull should have an outside coating made up of a number of possible materials, including fiberglass (also possible weaved in with the Kevlar), composites, polyethylene, and resin gel coat. Some manufacturers are taking Kevlar fibers and weaving them with fiberglass, which makes for a somewhat heavier but more durable canoe (but still typically lighter than Royalite and almost 1/2 the weight of full fiberglass). It is the easiest material to portage being very light weight. It is also very slippery which in part makes it extremely ideal for whitewater.
Kevlar in it's pure form is not the most ideal material for a canoe. Although it is very durable and can take shock very well, severe shock can crack a hull. Kevlar is very difficult to repair and the repairs are next to impossible to hide. The gel coat is easily abraded, and exposed Kevlar will shred out in fine fibers, next to impossible to repair. Materials blended with fiberglass are much better for extreme whitewater, and Kevlar is the material of choice for extreme paddlers.
Kevlar is very expensive and unless you plan to paddle in Class III+ or above, or plan to do frequent and long portages, if you are new to paddling, it is probably a case of over kill. However, if you can get a good Kevlar composite canoe for the cost of a Royalex one, you may do very well to consider the bargain that is available today, but be sure to invest in a gel coat repair kit and learn from some one who has used a repair kit in the past.
Fiberglass: Like aluminum, fiberglass has been around a long time as a material for canoes. The fiberglass canoes of twenty and thirty years ago have given way to a whole new breed of materials that are integrated with other fibers including Dacron and carbon fiber. A top of the line fiberglass canoe reinforced with Kevlar can be just as durable.
Fiberglass is more difficult to repair, but not as bad as Kevlar or aluminum. Repairs are easier to hide and the canoes tend to be very resistant to abrasion. S-glass and Gel-coat are the best materials for abrasion resistance. Like Kevlar, composite fiberglass is a good canoe for whitewater while basic fiberglass (which can be cheaper than polyethylene) is not. Fiberglass is not very resistant to shock, and a hull slammed up against rocks can crack. The outer materials are sensitive to sunlight and require indoor storage.
Fiberglass composite may be an excellent alternative to Royalex or Kevlar for whitewater. In it's pure form, it is best suited for limited use in flat water. Weight can vary from manufacturer and the composite blend used. Some fiberglass canoes can weight as much as an aluminum hull! If you are new to buying canoes, fiberglass is a difficult material to decide upon, and has a lot of variables. Just because a canoe is lightweight, does not mean it is high quality. Make sure to ask a lot of questions when considering a fiberglass canoe, and if you plan to whitewater, make sure to get a durable composite material.
Wood, Canvas, Cedar Strip, and Birch Bark Canoes: If money is no object, there can be a lot of satisfaction in owning a classic natural material canoe. Lightweight, wood, canvas, cedar strip, and birch bark canoes paddle like a dream, and will draw a lot of attention where ever you go. There hulls can be damaged very easily, and some designs require buoyancy chambers. Unless you have a trust fund, or have headed up five internet startups that have IPO'ed, you probably are not going to take a natural material canoe into whitewater. These canoes are ideal for flat water touring, and nothing can beat paddling in a remote area in a natural material canoe.
Natural material canoes are high maintenance, and do not do well to long term outside exposure. They are not the lightest materials (when compared to their very expensive synthetic counterparts), nor the most durable. They can be repaired very easily, but require training or in some cases craftsmen to issue repairs. Natural materials canoes are very expensive, and can cost over $4,000 US. Quality manufacturers are typically backlogged in production, as these canoes have to be hand built and the skill set required to make these beauties are in short supply.
Deciding on what style of boat...